To reduce the nitrogen oxide level in the exhaust gas in internal combustion engines used in motor vehicles, especially in diesel internal combustion engines, it is known that a catalytic converter device is provided for carrying out a selective catalytic reduction (SCR). Since ammonia is necessary for carrying out this reaction, for example, a urea/water solution is added as a reactant to the exhaust gas flowing in an exhaust system of an internal combustion engine by means of so-called injectors. Uniform mixing of the exhaust gas with reactant injected into same is necessary for an efficient catalytic reaction.